Electrical power and energy GCSE physics lesson: P = VI, P = I²R, E = Pt and E = QV

Electrical power: the virtual practical

A virtual experiment: the circuit follows a model, and it is not a practical in a lab. R1 is a 4 Ω heater. Record your readings first, then answer the questions about them.

1. Virtual practical. R1 is a 4 Ω heater. Change the cell's potential difference (B1) and record at least 4 readings, each at a different p.d. Include 6 V and 12 V. Before each one, predict the current.They set up a virtual circuit and record its meters' readings, each checked.
2. Use your reading at 6 V and P = V I. What is the heater's power, in watts?
3. Now use your current at 12 V and P = I² R, with R = 4 Ω. What is the heater's power, in watts?
4. From 6 V to 12 V the p.d. across the heater doubled. What happened to its power, and why?
   1. It doubled: power goes up as the p.d. goes up
   2. It stayed the same: the heater's resistance didn't change
   3. It became four times as big: the current doubled too
   4. It halved: more p.d. means less current
5. The heater is left at 12 V for 2 minutes. How much energy does it transfer, in joules?
6. How much charge flows through the heater in those 2 minutes, in coulombs? (Then work out E = Q V: does it agree with your last answer?)

Electrical power and energy: practice

Show your working on paper: what you know with its units, the equation, then the answer with its unit. Change minutes to seconds and kilowatts to watts first. The stretch is optional.

Diagnostic question: choose and explain

1. A current of 2 A flows through a 6 Ω resistor. What is the power transferred in the resistor?
   1. 12 W
   2. 24 W
   3. 3 W
   4. 144 W

Practice questions

1. A hairdryer on the 230 V mains takes a current of 5 A. What is its power, in watts?
2. A 60 W lamp is on for 5 minutes. How much energy does it transfer, in joules?
3. A current of 3 A flows through a 5 Ω resistor. What is its power, in watts?
4. A 2.3 kW kettle runs on the 230 V mains. What current does it take, in amps?
5. 40 C of charge flows through a lamp with a p.d. of 12 V across it. How much energy is transferred, in joules?
6. In a plug, which wire is at a p.d. of about 230 V?
   1. The live wire (brown)
   2. The neutral wire (blue)
   3. The earth wire (green and yellow)
   4. All three: they share the 230 V
7. Which transfers more energy: a 2 kW heater on for 10 minutes, or a 500 W heater on for 30 minutes?
   1. The 2 kW heater
   2. The 500 W heater: it is on three times as long
   3. They transfer the same
   4. You can't tell without the p.d.
8. UK mains electricity is a.c. at 50 Hz. What does that mean?
   1. The current flows one way only, as from a battery, at 50 A
   2. The current goes one way, then back, 50 complete times each second
   3. The p.d. of the supply is 50 V
   4. The current stops and starts 50 times each minute
9. A 12 V motor takes a current of 2 A for 5 minutes. How much energy does it transfer, in joules?
10. A heater with a resistance of 23 Ω runs on the 230 V mains. What is its power, in watts? (Find the current first.)
11. Stretch: an electric shower is rated 9.2 kW on the 230 V mains. What is its resistance, in ohms?

Support: Write down what you know with its unit, change minutes to seconds and kilowatts to watts, then choose the equation that has those letters and the one you want.

Electrical power and energy: exit

Write the equation first, then the numbers in watts and seconds.

1. A 1150 W microwave oven runs for 2 minutes 30 seconds. How much energy does it transfer, in joules?
2. A current of 2 A flows through a 15 Ω resistor. What is its power, in watts?

Exit check

An energy with time to change into seconds, and a power with a current to square, show who has it.

A 1150 W microwave runs for 2 minutes 30 seconds: the energy it transfers.

Electrical power and energy

For each calculation: write what you know with its units, change minutes to seconds and kilowatts to watts, write the equation, then the answer with its unit.